I'd like to share my experience on the subject.

I used to work with Testlink 1.6 to 1.8 and always had no trouble using XLS->XML->Testlink import.
But after upgrading to 1.9.1 I found a surprise. The good old import-to-Excel-put-data-and-export-XML-back didn't work since the steps feature emerged!
The problem was that Excel 2010 didn't handle that XML mapping well.
I tried many different ways but still didn't succeed. The new Import-From_Excel feature really did the import of headers and summaries but didn't care about steps and results. The task was to upload a large amount of old cases to run regression.
So, the best choice for me so far was to use good old TL 1.8 format and upload all the cases to TL 1.8... And then when it was done - I did migrate to 1.9

Hope that helps to anyone.